RAID-Related Terms

Definitions of RAID-related technologies.

Joel Sloss

August 1, 1997

2 Min Read
ITPro Today logo

RAID depends on several technologies to make it work, including SCSIcontrollers and wiring schemes that RAID and SCSI controllers use. Here aredefinitions of these technologies. For more information about SCSI technology,see Sean Daily, "SCSI and IDE," June 1997.

SCSI Standards


The original SCSI standard, offering synchronous (5MBps) andasynchronous (3MBps) modes. Supports single-ended configurations, with passiveand active termination.


The second standard, and the most common today. You can mixSCSI-1 and -2 devices on one SCSI-2 bus because it's backward-compatible toSCSI-1. SCSI-2 offers much higher burst and sustained transfer rates (althoughthe standard is only 5MBps), with a clock speed of 20MHz.

Fast SCSI-2

Runs at 10MBps.

Wide SCSI-2

Runs at 10MBps but extends the data path to 16 or 32bits.

Fast and Wide SCSI-2

Runs at 20MBps over a 16-bit data path.


Also known as Ultra SCSI (or Fast 20). It is fullybackward-compatible to SCSI-2 and uses 4-bit (deep) addressing. This newstandard extends performance to 40MBps.

Fast SCSI-3

Runs at 20MBps; cable length limits are the same asSCSI-2, unless you have differential wiring.

Wide SCSI-3

Runs at 40MBps. SCSI-3 is by nature "fast";doublewide uses 32 bits instead of 16.


Also known as Ultra 2. It is a 64-bit standard designed for64-bit PCI and runs at 80MBps with a clock speed of 40MHz.


Uses 4 bits instead of 3 to identify devices on the bus,allowing for up to 15 total devices on a channel.

Single-ended wiring

Cable wiring scheme that is more prone to noisebecause all signal lines use a common ground. It has a 6-meter cablelength limit.

Differential wiring

Wiring scheme that uses signal/ground wire pairs,which reduces noise and allows for up to 25-meter cables. The twoversions of differential are low-voltage (LVDS) and high-voltage (HVDS). The oneyou choose depends on the cable length you want, data speeds, and other factors­HVDSis for SCSI-4. You cannot mix single-ended and differential devices on the samebus.

Sign up for the ITPro Today newsletter
Stay on top of the IT universe with commentary, news analysis, how-to's, and tips delivered to your inbox daily.

You May Also Like